In IMS, users get their IP connectivity either from a home network or a visited network while users are roaming.
A UE (User Equipment) accessing IMS services requires an IP address that is logically part of the IMS IP Addressing Domain. This is established using an appropriate IP Connectivity Access Network (IP-CAN) bearer. Example of IP-CAN bearer is PDP (Packet Data Protocol) context of GPRS system. In GPRS system it is possible to connect to a GGSN (Gateway GPRS (General Packet Radio Service) Support Node) either in the VPLMN (Visited Public Land Mobile Network) or the HPLMN (Home Public Land Mobile Network). For routing efficiency this context may benefit from being connected through a GGSN in the visited network. The connection between the UE and the IMS (where the GGSN is either in the Home or the Visited network) is shown in FIGS. 1 and 2.
FIG. 1 shows the UE accessing IMS services with GGSN in the visited network, and FIG. 2 shows the UE accessing IMS services with GGSN in the home network.
Currently, users are able to choose either attach to visited P-CSCF (Proxy Call Session Control Function) or home P-CSCF, but not both at the same time. When a UE registers to its point of connection via visited P-CSCF to its home network the used roaming network is also checked and the operator of the home network is able to decide whether the user is allowed to roam to the visited network or not. Once registration is successfully completed, the UE in the visited network is allowed to perform a so called local breakout which is a network capability where user plane (media path) in a roaming case is not required to traverse via the home network, i.e. a user plane termination point is at the visited network.
For example, in the GPRS system shown in FIGS. 1 and 2, the GGSN of the visited network would be used for local breakout in FIG. 1.
In IMS, operators have capability to control which media are allowed for their users. Different IMS entities such as P-CSCF and S-CSCF (Serving CSCF) are able to inspect proposed media capabilities in SIP (Session Initiation Protocol) session setup procedures. If the proposed media capabilities are against operator policy and/or subscriber's user profile, IMS entities are able to reject a session attempt with SIP error response.